Keep your vocal cords well-hydrated before, during and after singing. The body must absorb water before it can be used, so drink water consistently. Remember that caffeine promotes dehydration so it's best to avoid it all together. Drink herbal tea or warm water with lemon to help limit mucus production.
Know that the littlest things affect the sound of your voice. Be conscious of your posture, when you hold your head a certain way, and the way you hold your mouth open as you expel sound. Pay close attention to your body and how it feels as you sing. When you know how your body is supposed to feel when you produce the proper sound, it becomes more natural and habitual.
Practice breath control. Fill your lungs so that when you place your hand on the middle of your ribcage, you can feel the movement at the very base of it, almost into your stomach area. If you can feel the expansion this low and not in the upper chest area, then you're breathing in the right place. Practice this until you can at least do 4 measures without breathing. Improve breath control with continued practice.
Develop a regular warm-up routine to use prior to singing. Some specific and helpful tips include breathing exercises to assist in air flow and preparation of diaphragm movement, humming your favorite song and performing some voice sirens to open up the vocal cords.
Warm up your voice with the accompaniment of piano scales or vocal exercises, or by using a professional voice warm-up sound track.
Relax and have fun. Singing is enjoyable for both the performer and the audience.